Patent Document 1 describes a spark-ignition direct-injection engine that injects fuel into a combustion chamber configured as a cavity, which is formed by recessing the top surface of a piston. This direct-injection engine accelerates combustion by adding ozone to intake air to be introduced into a cylinder. Flames spread from a central portion of the combustion chamber toward the periphery thereof as a spray of the fuel, injected through a fuel injection valve arranged at a bore center of the cylinder, scatters through the chamber. Before the flames reach the wall surface of the cavity, combustion is stopped. This forms a gas layer that does not contribute to combustion between a combustion gas in the central portion of the combustion chamber (i.e., in the cavity) and the wall surface of the cavity. This may significantly reduce the cooling loss. The direct-injection engine described in Patent Document 1 is further configured to have a high geometric compression ratio of higher than or equal to 15. This direct-injection engine improves the thermal efficiency significantly in cooperation with the above-described reduction in cooling loss.
The Applicant of this application proposed another technique for reducing the cooling loss in Patent Document 2. Specifically, in this technique, fuel is injected into a combustion chamber in the second half of a compression stroke, during which the fuel is injected in a specially designed manner. This forms an air-fuel mixture layer in a central portion of the combustion chamber, and also forms a heat-insulating gas layer around the air-fuel mixture layer, thus allowing the air-fuel mixture to be combusted in such a state. Note that the air-fuel mixture layer as used herein is a layer comprised of, and formed by, a combustible air-fuel mixture. The combustible air-fuel mixture may also be defined, for example, as an air-fuel mixture with an equivalence ratio φ of higher than or equal to 0.1. Forming, in the combustion chamber, the air-fuel mixture layer and the heat-insulating gas layer surrounding the air-fuel mixture layer allows the heat-insulating gas layer to reduce the contact between the combustion gas and the wall surface. Thus, the technique described in Patent Document 2 may reduce the cooling loss as significantly as the technique described in Patent Document 1.
According to the technique in which the air-fuel mixture layer and the heat-insulating gas layer surrounding the air-fuel mixture layer are formed in the combustion chamber, it is recommended that while a fuel injection valve is arranged such that its injection axis extends along the axis of a cylinder, a cavity symmetric with respect to the injection axis of the fuel injection valve be provided on a piston to face an injection tip of the fuel injection valve. The geometric compression ratio of an engine described in Patent Document 2 is also set to be higher than or equal to 15. The piston of the engine described in Patent Document 2 has a top surface, which has two inclined surfaces on intake and exhaust sides. Thus, the top surface is raised such that the two inclined surfaces form a triangular roof corresponding to a pent-roof-shaped surface of a cylinder head forming the ceiling surface of the combustion chamber. The cavity provided on a central portion of the top surface of the piston is formed by hollowing out portions of the inclined surfaces of the top surface of the piston.